1. Field of the Invention
The present invention relates to an antenna structure for portable radio equipment, and more specifically, to an antenna structure for portable radio equipment that allows inclining an antenna to a desired angle in order to improve sensitivity of the portable radio equipment.
2. Description of the Related Art
Existing types of transmitting/receiving antennas of portable radio equipment include a whip antenna mounted in a longitudinal direction of a cabinet and a helical antenna. The most often adopted antenna for portable radio equipment currently commercially available in Japan is a whip and helical antenna having a structure that operates as a whip antenna when pulled out and in that, when retracted, a helical antenna operates that is provided on a tip of the whip antenna with an insulating material intervening therebetween. In addition, an inverted F antenna or the like is built into the cabinet as an antenna for reception diversity.
In general, one factor that degrades antenna characteristic is antenna efficiency determined by an effective radiation area owing to the physical size and the structure of the antenna. Moreover, there is loss due to impedance mismatch with a radio unit and loss due to nonconformity of polarization plane and directivity in relation to an antenna that is a target of communication. Since the environment in which an antenna on mobile equipment of mobile communications such as portable radio equipment is installed or held constantly changes, directivity and plane of polarization vary constantly.
An antenna of a base station that communicates with portable radio equipment through transmission and reception is configured with vertical polarization as the main polarization. Transmission waves from a transmitting/receiving antenna of the base station are repeatedly reflected and diffracted by various structures such as buildings before they arrive at a receiving antenna of portable radio equipment. Vertical polarization, however, is generally known to be dominant as the main polarization of the waves arriving at portable radio equipment from the base station. In the reception characteristic of the transmitting/receiving antenna of the base station, it is obvious from the reversibility of the transmission/reception characteristic of the antenna that the greater the vertical polarization component in the waves arriving from portable radio equipment, the higher the gain of the antenna becomes.
Whip antennas and the like formed on portable radio equipment have radiation patterns of various forms depending on an electrical length of an antenna element and a dimension of the cabinet. When the portable radio equipment is provided such that it stands upright, the main polarization is known to become vertical polarization in relation to the ground.
Thus, during a call on portable radio equipment (often used with an inclination of about 60xc2x0 from the direction of the zenith) or when the portable radio equipment is placed horizontally on a desk and the like, nonconformity in the direction of main polarization becomes significant between the antenna formed on the portable radio equipment and the antenna of the base station that has vertical polarization in relation to the ground, and the gain of the antenna is disadvantageously degraded.
FIGS. 6A and 6B are diagrams related to the description of polarization loss when portable radio equipment is inclined. As shown in FIG. 6A, Excex8 indicates an electric field level of vertical polarization and Excfx86 indicates an electric field level of horizontal polarization when portable radio equipment 1 is erected vertically. As shown in FIG. 6B, when portable radio equipment 1 is inclined by 60xc2x0, the electric field level of vertical polarization becomes Excex8xe2x80x2 and the electric field level of horizontal polarization becomes Excfx86xe2x80x2, producing a polarization loss as indicated by the following expression:
Polarization loss=20 log10(Excex8/Excex8xe2x80x2)dB.
In order to reduce such polarization loss, conventionally, techniques are proposed of providing a metal joint portion, for example, in order mechanically to incline the antenna alone, for instance, in Japanese Patent Laying-Open No. 8-274525, Japanese Patent Laying-Open No. 6-260959, and Japanese Patent Laying-Open No. 5-55817.
These mechanisms, however, may disadvantageously involve an increase in the number of parts and may easily break when portable radio equipment 1 is dropped.
Thus, the principal object of the present invention is to provide an antenna structure for portable radio equipment that can be formed with a small number of parts, that allows inclining of the antenna to a desired angle, and that can reduce loss due to nonconformity of polarization.
The present invention is an antenna structure mounted on a cabinet 15 of portable radio equipment including an antenna made of a conductor of elastic alloy and an inclining portion provided on a cabinet for inclining the antenna.
Preferably, the antenna has vertical polarization and the inclining portion inclines a polarization plane of the antenna to a direction vertical to the ground.
Preferably, the inclining portion includes a sliding member whose tip engages a lower portion of the antenna within the cabinet and which slides to incline the antenna.
Preferably, the inclining portion includes a stopping member whose upper end engagingly stops against the antenna outside the cabinet and which inclines the antenna.
Preferably, the inclining portion includes a disk member having a groove that is cut away departing from a center, and a lower portion of the antenna is inserted in the groove of the disk member and the disk member is rotated to incline the antenna.
Another aspect of the present invention is an antenna structure mounted on a cabinet of portable radio equipment, characterized in that a polarization plane of an antenna having vertical polarization during a call and having a length of xcex/2 is inclined to a direction vertical to the ground.